Single Cab Bass??

Hey everyone I have two 10 inch subs in sealed boxes and they slam in my single cab. But I want to know how to get louder without being ported or another box, because I have absolutely NO ROOM for a ported box that's tuned below 40 hz, and I don't want it tuned that high. If I roll downt he windows its really loud, but with the windows up the bass sounds flat. How can I help this??